1. Field of the Invention
The invention pertains to a liquid crystal display device, and more particularly, to an array substrate having a color filter on a thin film transistor structure and a method for fabricating the same. Although the invention is suitable for a wide range of applications, it is particularly suitable for increasing the aperture ratio and simplifying the fabrication process.
2. Description of the Related Art
Flat panel display devices are thin, light weight, and have low power consumption, and they have been used for portable display devices. Among the various types of flat panel display devices, liquid crystal display (LCD) devices find wide use in laptop computers and desktop computer monitors because of their superiority in resolution, color image display, and display quality.
Optical anisotropy and polarization characteristics of liquid crystal molecules are utilized to generate desirable images. Liquid crystal molecules have specific alignment directions that result from their own peculiar characteristics. The specific alignment directions can be modified by electric or magnetic fields that are applied to the liquid crystal molecules. In other words, an electric fields applied upon the liquid crystal molecules can change the alignment of the liquid crystal molecules. Due to the optical anisotropy, the incident light is refracted according to the alignment of the liquid crystal molecules.
Specifically, the LCD devices include upper and lower substrates having electrodes that are spaced apart and face into each other. A liquid crystal material is interposed between the substrates. Accordingly, when a voltage is applied to the liquid crystal material through the electrodes of each substrate, the alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage, thereby displaying images. By controlling the applied voltage, the LCD device varies the light transmittances to display image data.
Liquid crystal display (LCD) devices find wide applications in office automation (OA) and video equipment due to their characteristics such as light weight, slim design, and low power consumption. Among different types of LCD devices, active matrix LCDs (AM-LCDs) having thin film transistors and pixel electrodes arranged in a matrix form provide high resolution and superior moving images. A typical LCD panel has an upper substrate, a lower substrate, and a liquid crystal layer interposed between the substrates. The upper substrate (referred to as a color filter substrate) includes a common electrode and color filters. The lower substrate (referred to as an array substrate) includes thin film transistors (TFT's), such as switching elements, and pixel electrodes.
As previously described, an LCD device operates on the principle that the alignment direction of liquid crystal molecules varies with applied electric fields between the common electrode and the pixel electrode. Accordingly, the liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon the polarity of the applied voltage.
FIG. 1 shows an expanded perspective view illustrating a related art active matrix liquid crystal display device. As shown in FIG. 1, the LCD device 11 includes an upper substrate 5 (referred to as a color filter substrate) and a lower substrate 22 (referred to as an array substrate) sandwiching a liquid crystal layer 14. On the upper substrate 5, a black matrix 6 and a color filter layer 8 form an array matrix including multiple red (R), green (G), and blue (B) color filters surrounded by the black matrix 6. Additionally, a common electrode 18 is formed on the upper substrate 5 and covers the color filter layer 8 and the black matrix 6.
On the lower substrate 22, thin film transistors T are formed in an array matrix corresponding to the color filter layer 8. Gate lines 13 and data lines 15 perpendicularly cross one another such that each TFT T has a location adjacent to each intersection of the gate lines 13 and the data lines 15. Furthermore, pixel electrodes 17 are formed on a pixel region P defined by the gate lines 13 and the data lines 15 of the lower substrate 22. A transparent conductive material having high transmissivity, such as indium tin oxide (ITO) or indium zinc oxide (IZO), forms the pixel electrode 17.
FIG. 1 also shows a storage capacitor C disposed to correspond to each pixel P and connected in parallel to each pixel electrode 17. The storage capacitor C includes a portion of the gate line 13 as a first capacitor electrode, a storage metal layer 30 as a second capacitor electrode, and an interposed insulator (shown as reference numeral 16 of FIG. 2). Since the storage metal layer 30 connects to the pixel electrode 17 through a contact hole, the storage capacitor C electrically contacts the pixel electrode 17.
The related art LCD device shown in FIG. 1 operates by applying a scanning signal to the gate electrode of the thin film transistor T through the gate line 13, and a data signal is applied to the source electrode of the thin film transistor T through the data line 15. As a result, the liquid crystal molecules of the liquid crystal material layer 14 align and arrange by the operation of the thin film transistor T, and this alignment controls the incident light passing through the liquid crystal layer 14 to display an image. Namely, the electric fields induced between the pixel and common electrodes 17 and 18 re-arrange the liquid crystal molecules of the liquid crystal material layer 14 so that the incident light can accordingly be converted into the desired images.
When fabricating the LCD device 11 of FIG. 1, the upper substrate 5 is aligned with and attached to the lower substrate 22. In this process, the upper substrate 5 may misalign with respect to the lower substrate 22, and light leakage may occur in the completed LCD device 11 due to a marginal error in attaching the upper and lower substrates 5 and 22.
FIG. 2 is a schematic cross-sectional view taken along line II-II of FIG. 1, illustrating a pixel of the related art liquid crystal display device.
As shown in FIG. 2, the related art LCD device includes the upper substrate 5, the lower substrate 22, and the liquid crystal layer 14. The upper and lower substrates 5 and 22 are spaced apart from each other, and the liquid crystal layer 14 is interposed between the substrates. The upper and lower substrates 5 and 22 are often referred to as a color filter substrate and an array substrate, respectively, because the color filter layer 8 is formed upon the upper substrate, and array elements, such as TFT and pixel electrodes, are formed on the lower substrate 22.
In FIG. 2, the thin film transistor T is formed on the front surface of the lower substrate 22. The thin film transistor T includes a gate electrode 32, an active layer 34, a source electrode 36, and a drain electrode 38. Between the gate electrode 32 and the active layer 34, a gate insulation layer 16 protects the gate electrode 32 and the gate line 13. As shown in FIG. 1, the gate electrode 32 extends from the gate line 13 and the source electrode 36 extends from the data line 15. All of the gate, source, and drain electrodes 32, 36, and 38 are formed of a metallic material while the active layer 34 is formed of silicon. A passivation layer 40 protects the thin film transistor T. In the pixel region P, the pixel electrode 17 formed of a transparent conductive material is disposed on the passivation layer 40 and contacts the drain electrode 38 and the storage metal layer 30.
Meanwhile, as mentioned above, the gate electrode 13 acts as a first electrode of the storage capacitor C and the storage metal layer 30 acts as a second electrode of the storage capacitor C. The gate electrode 13 and the storage metal layer 30 thus constitute the storage capacitor C along with the interposed gate insulation layer 16.
FIG. 2 also shows the upper substrate 5 that is spaced apart from the lower substrate 22 and is also over the thin film transistor T. On the rear surface of the upper substrate 5, a black matrix 6 is disposed in a position corresponding to the thin film transistor T, the gate line 13 and the data line 15. The black matrix 6 is formed on the entire surface of the upper substrate 5 and has openings corresponding to the pixel electrode 17 of the lower substrate 22, as shown in FIG. 1. The black matrix 6 prevents a light leakage in the LCD panel except for the portion for the pixel electrode 17. The black matrix 6 protects the thin film transistor T from the light in order to prevent generation of a photo-current in the thin film transistor T. The color filter layer 8, which is formed on the rear surface of the upper substrate 5, covers the black matrix 6. Each of the color filters 8 has one of the red 8a, green 8b, and blue 8b colors and corresponds to one pixel region P where the pixel electrode 17 is located. A common electrode 18 formed of a transparent conductive material is disposed on the color filter layer 8 over the upper substrate 5.
In the related art LCD panel discussed above, the pixel electrode 17 has a one-to-one correspondence with one of the color filters. Furthermore, in order to prevent cross-talk between the pixel electrode 17 and the gate and data lines 13 and 15, the pixel electrode 17 is spaced apart from the data line 15 by a distance A and from the gate line 13 by a distance B, as shown in FIG. 2. The open spaces A and B between the pixel electrode 17 and the data and gate line 15 and 13 cause malfunctions such as light leakage in the LCD device. Namely, the light leakage mainly occurs in the open spaces A and B so that the black matrix 6 formed on the upper substrate 5 should cover the open spaces A and B. However, when the upper substrate 5 is arranged with the lower substrate 22 or vice versa, a misalignment may occur between the upper substrate 5 and the lower substrate 22. Therefore, the black matrix 6 is extended to completely cover the open spaces A and B. That is, the black matrix 6 is designed to provide an aligning margin to prevent a light leakage. However, when extending the black matrix, the aperture ratio of a liquid crystal panel reduces as much as the aligning margin of the black matrix 6. Moreover, if there are errors in the aligning margin of the black matrix 6, light leakage still occurs in the open spaces A and B, and deteriorates the image quality of an LCD device.